Surface-stabilized Ferroelectric Liquid Crystal Devices Research Articles

Fast Visible-Near Infrared Switchable Liquid Crystal Filter

A conventional liquid crystal display consists of a liquid crystal cell between two crossed polarizers. By applying a voltage, the liquid crystal can be switched and the transmission of light can be manipulated. For thick devices, the transmission shows a strong wavelength dependency and this can be used to construct a tunable Lyot-Öhman filter. Such a filter consists of several liquid crystal cells with polarizers in between with a narrow transmission band that can be shifted over a certain wavelength range. Instead of aiming for a narrow transmission peak, in this work the aim is to achieve a wide transmission wavelength range. In this way the transmission band can be switched, for example, from the visible wavelength range to the near-infrared range. Because the device needs only two different states, bistable surface-stabilized ferroelectric liquid crystal devices can be used which offer short switching times in the order of 100 μs. Numerical optimization of this switch reveals that high contrast ratios can be achieved by using 2 fixed and 2 switchable retarders.

Shift of the Hysteresis Inversion Frequency with Temperature in a Surface Stabilized Ferroelectric Liquid Crystal Cell

The capacitance–voltage (C–V) hysteresis inversion frequency ( fi) of a surface stabilized ferroelectric liquid crystal device (SSFLC) is found to shift to the higher frequency side with increasing temperature. The frequency ( fc) up to which the amount of switchable polarization (ASP) decreases also shifts to the higher value. ASP for a fixed frequency also decreases with increasing temperature. The decrease in the tilt angle, spontaneous polarization and the traversable path of the chiral molecule between two fully switched states on the smectic cone, with increasing temperature causes the decrease in ASP and also shifts fi. Accordingly, temperature dependence of the inversion of the dielectric biaxiality can be determined.

A defect free bistable Cl SSFLC devices

Recent progress in both low pretilt and high pretilt defect free Cl surface stabilized ferroelectric liquid crystal (SSFLC) devices is reviewed. First, by numerical calculation to investigate the balance between surface azimuthal anchoring energy and bulk elastic energy within the confined chevron layer geometry of C1 and C2, it is possible to achieve a zigzag free C1 state by low azimuthal anchoring alignment with a low pretilt angle. The critical azimuthal anchoring coefficient for defect free C1 state is calculated. Its relationship with elastic constant, chevron angle as well as surface topography effect are also discussed. Second, using 5° oblique SiO deposition alignment method a defect free, large memory angle, high contrast ratio and bistable C1 SSFLC display, which has potential for electronic paper applications has also been developed. The electro‐optical properties and bistability of this device have been investigated. Various aspects of defect control are also discussed.

Director tilting of liquid crystals on photoisomerizable polyimide alignment layers doped with homeotropic surfactant

Director tilting of liquid crystal (LC) molecules on photosensitive alignment layers has been investigated. The layers, polyimides with photoisomerizable groups in their main chains, were irradiated with unpolarized near-UV (UPUV) light at oblique incidence. The irradiated film was found to align LC molecules homogeneously with a negligible pretilt angle. It was found that the pretilt angle can be generated successfully by introducing surfactant lecithin and decreasing the incidence angle of the UPUV light. The LC alignment and electro-optical properties of the LC cells with the photoalignment layers were demonstrated to be suitable for twisted nematic LC, vertical alignment LC, and surface-stabilized ferroelectric LC devices.

Inverse switching phenomenon of surface-stabilized ferroelectric liquid crystals under a high voltage region

A study of the electro-optie response of a surface-stabilized ferroelectric liquid crystal device has been made for the high drive voltage region. We find an inverse switching memory effect in the polarity to be built up after the trailing edges of the applied voltage pulses. The inverse switching memory effect is observed by applying higher and longer voltage pulses. The effect is considered to be produced by the internal electric field produced by ions, and the internal electric field strength depends on the thickness of the orientational film.

Threshold Properties of SSFLC in Terms of Polarization Structures

Bistability of the surface stabilized ferroelectric liquid crystal (SSFLC) is one of the most important factors of display perfomances. The electrooptical switching is based on the bistability which provides threshold characteristics to the SSFLC display device. The threshold properties of the SSFLC device which has a layer structure of liquid crystal molecules and provides an in-plane molecular switching are assumed to be governed by not only a surface anchoring but also a bulk structure itself. We show some experimental results which suggest a contribution of ferroelectric liquid crystal's bulk structure to the electrooptical threshold properties. Layer structures: the bookshelf and the chevron geometries are one of the main bulk factors. Our experimental results clarified that the polarization switching behavior was intrinsically different between the bookshelf and the chevron structures. The bookshelf geometry also shows a possibility of gray shades.

Switching Behavior of Surface Stabilized Ferroelectric Liquid Crystal Devices with Chevron Layer Structure

Abstract The switching behavior in the C1-uniform (C1U), C1- twisted C1T) and C2-uniform (C2U) states, which were observed in surface-stabilized ferroelectric liquid crystal (SSFLC) devices with the chevron layer structure, were investigated. The voltage dependence of the extinction angle was investigated in the C1U and C2U states and was discussed in terms of the switching model of these states. The boat-shape disclination loops were observed in the C1U, C1T and C2U states, suggesting that they were attributed to the molecular switching at the chevron interface. In addition, this paper describes a gray scale with smooth voltage-transmission characteristic, which were induced by the voltage dependence of the extinction angle.

Flow Coupled Switching Equations for Surface-Stabilized Ferroelectric Liquid Crystal Cells

Flow-coupled switching equations are derived for surface-stabilized ferroelectric liquid crystal devices, starting from Leslie, Stewart and Nakagawa's continuum theory for smectic C liquid crystals. Comparisons with nematic like dynamic equations are made.

Novel Switching with Gray Scale in Surface-Stabilized Ferroelectric Liquid Crystal Devices

The apparent tilt angle in surface-stabilized ferroelectric liquid crystals (SSFLCs) is dependent on the applied electric field for monostable and bistable devices. This behavior is attributed to the fact that the director profile of molecules in SSFLC cells is changed by an interaction between spontaneous polarization and the electric field. Based on this behavior, the transmission in SSFLCs can be controlled by the applied voltage, resulting in a smooth voltage-transmission curve.

Polarization-independent, broadband, bistable, 2 × 2 optical exchange switch

A polarization-independent, broadband, bistable, 2 × 2 optical exchange switch utilizing a surface-stabilized ferroelectric liquid-crystal device is presented. The switch has <−15-dB cross talk over a 50-nm optical bandwidth, with a minimum of −18 dB at 615 nm. By modeling the individual components the theoretical output of the device is obtained, which agrees well with experimental results. Four independent 2 × 2 switches incorporated into one single compact device are also demonstrated. The typical switching time for these switches is ∼150 μsec.

Optical Methods for the Measurement of Layer-Tilt and Pretilt Angles in Surface-Stabilized Ferroelectric Liquid Crystal Devices

We have used simple optical methods to determine, simultaneously, the layer-tilt, pretilt, quiescent state azimuthal, and tilt angles of surface-stabilized ferroelectric liquid crystal (SSFLC) devices as a function of temperature. In a SSFLC cell aligned by rubbed polymer films, the measured pretilt angles are small and the layer-tilt angle is, in general, less than the tilt angle for the same material at any given temperature in the smectic C phase. The measured pretilt, layer-tilt, angles show a common tendency to converge to zero as the temperature approaches the smectic C-to-A transition, while the azimuthal angle of the director in the stable quiescent state increases toward 90°. We observed that the direction of rubbing for alignment was parallel to the projection of the layer-normal onto the substrate surface within the temperature range of the smectic C and smectic A phases.

Electro-optic applications of ferroelectric liquid crystals to optical computing

Abstract The surface-stabilized ferroelectric liquid crystal (SSFLC) electro-optic device is superior in many regards to other available optical and electro-optical switching devices for optical computing applications requiring the parallel operation of a large number of elements. The SSFLC device's large interaction with light, stemming from its large permanent birefringence, allows individual electro-optic elements to be small in size and hence allows the fabrication of many-element arrays on substrates of size convenient for optical systems. Further, its low switching energy allows these dense arrays to be operated at high speeds without unmanageable power dissipations, while its bistability allows its use as a high speed memory. We examine the fundamental physical limitations to the performance of the SSFLC device in parallel applications, and show that million element devices with microsecond switching times are feasible, allowing terabit/second data throughput rates. We also present prototype optica...

Optical interconnection network using polarization-based ferroelectric liquid crystal gates

A polarization-based 4 x 4 optical interconnection network using surface stabilized ferroelectric liquid crystal (SSFLC) gates is demonstrated. The SSFLC gates are comprised of an SSFLC device sandwiched between two polarizing beam splitters. Optical crosstalk using these fast switching programmable devices can be limited to ~-20 dB/gate, which would allow 2-D interconnection networks to be fabricated with thirty-one input channels and 3-D interconnection networks with approximately 225 input channels.